Recently, due to the increase in prices of petroleum recources, studies have been out on collecting oils from oil sand layers buried under the ground in areas such as Canada and Venezuela. The oil sand is generally present as a layer about 50 m thick and is underground at a depth of several hundred meters. However, oils in the oil sand can not be collected and extracted at a normal temperature due to their high viscosity. Therefore, at the present time, methods of extracting oils comprise raising the temperature of oils by injecting heated steam into the oil sand layer to reduce the viscosity of oils. However, such a method is not efficient due to the increase in cost. Accordingly, studies have been carried out to provide a method of collecting oils with high productivity. One method involves the use of two conduct pipes (steel pipes or stainless steel pipes) having each an electrode part on the end thereof which are buried at an interval of about 30-200 m. Accordingly, the electrode part is situated on the oil sand layer, and an electric potential of several hundred volts to several ten thousand volts is applied to both electrodes. This causes the temperature of the oil sand layer to rise due to joule's heat which reduces the viscosity of the oils.
The specific resistance of the oil sand layer is several times higher than that of the upper strata. Accordingly, it is necessary for the conduct pipes to be covered with an electrically insulating material over the portion which is buried in the strata so that the electric current does not flow through the upper strata. If the conduct pipes are not covered with an electrically insulating material, the electric current does not flow between electrodes buried in the oil sand layer, because it flows through the stratum part. Accordingly, there has been a significant need to develop conduct pipes covered with an electrically insulating material which are cappable of being used under such specific conditions.
Characteristics required for this electrical insulating material include
(A) the ability to withstand a voltage of several hundred volts-several ten hundred volts and a volume resistivity of at least 10.sup.6 .OMEGA.-cm at not only a normal temperature but also at a temperature capable of reducing the viscosity of oils in the oil sand layer (about 300.degree. C.), PA1 (B) the ability to withstand hot water (about 300.degree. C.), because water contained in the oil sand layer is heated to a temperature capable of reducing the viscosity of the oil sand layer (about 300.degree. C.), and PA1 (C) it has mechanical strength capable of hanging the electrode and mechanical impact strength of such a degree that it is not broken by contacting wall of the pit when burying the electrode hanging on the end of the conduct pipe in the oil sand layer.